The present invention relates to the batch plating of a long metal lead frame and more particularly to immersion plating on a reel.
The term immersion plating is used in the plating art and is employed herein to mean plating without an applied voltage and is accomplished at least in part by the substitution of the metal to be plated by metals in the plating bath. Immersion plating is further characterized by an inherent self limiting of the plating thickness, usually no greater than 100 microinches.
Lead frames are commonly employed in the production of electronic parts such as capacitors, inductors and transistors.
Prior to actual component assembly to a lead frame, an intermediate plating process is normally required. The usual lead frame base metal is copper or Kovar (a tradename of Westinghouse Electric Co., Blairsville, Pennsylvania). These metals are commonly plated with systems of nickel, gold, silver and in some cases tin or tin-lead alloys. These metals are plated onto the lead frame to allow subsequent component assembly operations such as eutectic semiconductor chip bonding, wire welding and/or soldering operations. The plating operation also protects the electronic components from the effects of corrosion from the lead frame base metal.
The plater normally uses the reel that the punched lead frame is wound on as a "supply" reel. The actual plating is done while the lead frame is unwound from the "supply" reel to a "takeup" reel. The mechanisms for transporting the lead frame during plating are quite sophisticated, involve substantial capital expense and require the attention of skilled operators.
Batch plating such as plating on a reel has heretofore been considered impractical particularly since the adjacent turns in a wound metal strip touch each other and effectively mask random areas from the plating solution. Otherwise batch plating requires only comparatively modest expenditures (about 1/20) for the necessary capital equipment, elimination of a source of physical distortion of the lead frame, and offers potentially reduced processing costs. A batch plating system also readily allows plating more than one long lead frame strip in the same tank at the same time.
It is therefore an object of this invention to provide a method for uniformly batch plating a long lead frame strip that is wound on a reel.
It is a further object of this invention to provide a low cost plating method for a long lead frame requiring a minimum of capital equipment.
It is yet a further object of this invention to provide a method for batch plating a plurality of long lead frame strips at once in a common plating solution.